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In In Vitro Fertilization (IVF) process there is ovarian stimulation, followed by the retrieval of the eggs and fertilization. From this, there are usually many embryos than can be transferred into the uterus (womb) at once. Our goal is to have a healthy pregnancy. Therefore, we only transfer an appropriate number of embryos to reduce the chance of a higher order of multiple pregnancies and its inherent risks. In an IVF treatment cycle, a patient may have extra embryos after embryo transfer that can be frozen (cryopreserved) and stored.
Embryo freezing provides the opportunity to conceive more than once from a single egg retrieval cycle. It involves laboratory techniques (cryopreservation) that allow us to store the embryos in liquid nitrogen and they can remain there for several years. No study has shown any increased risk of birth defects in babies conceived using frozen embryos compared with babies born of naturally conceived pregnancies from mothers of the same age bracket. In addition, age‐related pregnancy success rates and birth defect risks correspond to the age of the eggs themselves (determined by the age of the patient when the eggs were fertilized) and not to the age of the patient when they were transferred into the uterus.
Preparation for the transfer of frozen embryos involves the use of hormone medications (oestrogen and progesterone) in sequence to produce a suitable lining in the uterus, which allows the embryos to implant. Only good embryos are suitable for freezing, but even with this pre‐selection, it is possible that some, and rarely all, will not survive the freezing and thawing process. Only at the time of expected embryo transfer are patients informed about the number of embryos that have survived the thawing process. Unfortunately, there is no way of knowing this before the time. Pregnancy rates following the transfer of frozen embryos are similar to those following the transfer of fresh embryos.